Author: (A. Adams & Reeve, 1850)

Order: Anaspidea  Family: Aplysiidae

Maximum Size: 100 mm

Sightings: Sunshine Coast

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Syphonota geographica (A. Adams & Reeve, 1850)

A circumtropical and warm temperate species of Anaspidea sea slug – a sea hare.

This species will become known as Aplysia geographica once the molecular sequencing work by Nimbs, 2020 (unpublished data), is published in the required format. That data shows this species to be firmly nested in Aplysia, sister to Aplysia kurodai and Aplysia sydneyensis.

Syphonota geographica is a medium to large species, up to 100 mm, with an elongate oval body shape that is high in profile (though not as high as some Aplysia) when moving but lower when at rest. The body foot and parapodia are smooth. The parapodia (extensions of the foot) are large and free to move their whole length (unfused) and may be carried high and undulating or closed tightly down upon the mantle. Beneath the mantle it possesses a thin and ovate plate-like internal shell. There is a conspicuous posterior siphon exiting the mantle cavity, being large and long extending well past the edges of the parapodia even when they are tightly drawn together. The gill is located in the mantle cavity and is unobservable. The head is elongated and the antero-lateral head tentacles, or oral tentacles, are large, enrolled and fimbriated. Together with the sloping head, they also perform as a broad shovel to facilitate burrowing. Further posteriorly, dorsally, higher up on the head, at the commencement of the parapodial lobes, are the rhinophores. These are enrolled, relatively small and closely sited.

Colour is variable from bright green to dull green to even brownish with a patchwork of markings, contrasting in white or cream, that have given rise to its specific epithet of geographica, due to the map-like resemblance they produce. There is also considerable variability in the appearance of this patterning. The ventral surface of the foot ranges from bright yellow to a dull greenish yellow.

Observed to mate in chains, like other sea hares, the anterior-most acting as a female the hind-most a male with all those in-between acting as both male and female concurrently. The produced spawn is of a tangled-string appearance.

Diet has been recorded as sea grass of the genus Halophila that it crops off in large pieces to be mechanically reduced by the cuticularized gizzard of the digestive system.

It moves using its muscular foot in a creeping motion but has also been observed to swim vigorously by strong flapping, undulating motions of the large, essentially free, parapodia. (See video below for this action.)

It is active and feeds nocturnally, burying itself in the sandy substrate during the day. Buried or partially buried specimens are sometimes observed above the low tide line, during daylight hours, the moist sand providing protection and enabling survival until the next tidal inundation. Those that are only partially buried are so as a result of the inability to manipulate the heavy drying sand sufficiently to cover itself. This behaviour has been postulated to provide protection from predators, anchoring against strong tidal currents and prevention of desiccation should it be caught out above the dropping tide line.

From time to time it may be exceedingly abundant in certain areas due to conditions and yet may not be seen again for years at a time.

It has the typical sea hare presentation and has been described as an Aplysia modified for a burrowing lifestyle. (This suggestion along with “It bears no internal anatomical differences that would preclude it from the genus Aplysia.” [Nimbs et al, 2016] being borne out by the subsequent molecular sequencing results.) Features adapted for this lifestyle include: the rhinophores being set further back than normal, towards the parapodia (from which they may receive some protection from abrasion), sited much closer together and smaller than those of its relatives; the mantle closely enveloping the small mantle cavity with its large posterior siphon extending past the edges of the parapodia; and the ability to flatten itself.

Originally described as Siphonotus geographicus it was relocated, in 1854, to the newly erected Syphonota H. Adams & A. Adams, 1854, Siphonotus already being occupied by a genus of millipedes.

The species has subsequently been described several times under different names creating a number of other synonyms including: Aplysia piperata E. A. Smith, 1884; Aplysia mouhoti Gilchrist,1895 and Aplysia scripta Bergh, 1905. At one time it was placed in Paraplysia.

Just as with a number of other species originally considered to have a wide multi-ocean distribution, it may transpire that Syphonota geographica conceals a species complex, once molecular sequencing of specimens from different localities is undertaken.

Above: Syphonota geographica flapping its parapodia as it moves across the floor of a shallow tidal lagoon. The opening parapodia reveal the characteristic small mantle and the distinctive large posterior siphon.

David A. Mullins – October 2022

References:
– Adams, A. & Reeve, L. A. (1850). Mollusca, Part 3. In A. Adams (ed.), The zoology of the voyage of H.M.S. Samarang, under the command of Captain Sir Edward Belcher, C.B., F.R.A.S., F.G.S., during the years 1843-1846. Reeve & Benham, London.

– Adams, H. & Adams, A. (1853-1858). The genera of Recent Mollusca; arranged according to their organization. London, van Voorst. [Vol. II: 1-92 (1854).]

– Smith, E. A. (1884). Mollusca. pp. 34-116; 487-508 in: Report on the zoological collections made in the Indo-Pacific Ocean during the voyage of H. M. S. Alert 1881-2. Part I. The collections from Melanesia; Part II. Collections from the western Indian Ocean. London: The Trustees [of the British Museum (Natural History)].

– Bergh, L. S. R. (1905). Die Opisthobranchiata der Siboga-expedition. Siboga-Expeditie. 50: 1-248, pls 1-20.

– Eales, N. B. (1960). Revision of the world species of Aplysia (Gastropoda, Opisthobranchia). Bulletin of the British Museum (Natural History), Zoology, 5(10): 267-404.

– Thompson, T. E. (1976). Biology of Opisthobranch Molluscs. Volume 1. The Ray Society, London.

– Bebbington, A. (1977). Aplysiid species from eastern Australia with notes on the Pacific Ocean Aplysiomorpha (Gastropoda, Opisthobranchia). Transactions of the Zoological Society, London, 34: 87- 147.

– Rudman, W. B., (1999 May 26) Syphonota geographica (Adams & Reeve, 1850). [In] Sea Slug Forum. Australian Museum, Sydney. Available from http://www.seaslugforum.net/factsheet/syphgeog and associated messages.

– Klussmann-Kolb, A. (2004). Phylogeny of the Aplysiidae (Gastropoda: Ophisthobranchia) with new aspects of the evolution of sea hares. Zoologica Scripta 33: 439–462.

– Nimbs, M. J., Willan, R. C. & Smith, S. D. A. (2016). Is Port Stephens, eastern Australia, a global hotspot for biodiversity of Aplysiidae (Gastropoda: Heterobranchia)?. Molluscan Research. 37(1): 47-65.

– Nimbs, M. J. & Smith, S. D. A. (2017). Revision of the southern distribution limit for the tropical marine herbivore Syphonota geographica (A. Adams & Reeve, 1850) (Heterobranchia: Aplysiidae) in a global climate change hot-spot. Australian Zoologist, 38, 582–589.

– Nimbs, M. J, Willan, R. C., & Smith, S. D. A. (2017). An historical summary of the distribution and diet of the Australian sea hares (Gastropoda: Heterobranchia: Aplysiidae). Zoological Studies, 56, 1–15.

– Nimbs, M. J. (2020). The taxonomy and systematics of the Australian Aplysiidae (Mollusca: Gastropoda). (Doctoral dissertation, Southern Cross University).

– MolluscaBase eds. (2022). MolluscaBase. Syphonota geographica (A. Adams & Reeve, 1850). Accessed through: World Register of Marine Species at: https://www.marinespecies.org/aphia.php?p=taxdetails&id=370566